This invention relates to a dry refractory (i.e., a monolithic refractory installed in dry powder form without the addition of water or liquid chemical binders), particularly a dry refractory suitable for use in metal/heat containment and thermal insulation applications that provides superior resistance to crack propagation.
Refractories are widely used as working linings and secondary (safety) linings in the metal processing and related fields. These refractory linings contain molten metal and slag in metal processing and transfer vessels. Some refractory linings also are used to contain the heat and gases associated with metal processing operations within the vessels. As used herein, a “metal containment” application is one in which containment of molten metal and slag is of primary or even sole importance, while a “metal/heat containment” application is one in which both heat containment (insulation) of the vessel and containment of molten metal and slag are of interest.
Refractory linings for metal and heat/metal containment applications typically are consumable. They erode, crack, or otherwise are damaged by exposure to conditions within the vessel. When a certain amount of consumption of or damage to the refractory lining has occurred, repair or replacement of the lining is needed. Repair or replacement interrupts the metal processing operation, sometimes for an extended time. Some interruptions are unexpected while others are more or less predictable. Because repair or replacement of a refractory lining disrupts operations, it is desirable that the refractory lining perform in a predictable manner to allow for scheduled rather than emergency repairs.
Erosion of the refractory lining due to contact with the corrosive molten metal and slag results in a gradual consumption of the refractory lining. Erosion rates generally can be predicted by visual inspection of exposed portions of the vessel lining or other techniques. A predictable erosion rate can be established for a particular refractory lining based on the metal and thermal containment characteristics of the application and historical refractory consumption. For electric induction furnaces, the erosion rate can also be estimated by changes in electrical readings over time.
Cracking of a refractory lining results when a bonded, brittle refractory is subjected to thermal and mechanical stresses. These stresses commonly result from expansion and contraction of the lining as a result of changes in the thermal environment. Cracking allows molten metal and slag to infiltrate into the lining, resulting in an isolated failure area in the metal processing or transfer vessel. Failure of a refractory lining due to cracking is much less predictable than erosion. Cracks often do not occur in an exposed area of the refractory lining so visual inspection usually is not helpful in identifying cracking. The nature of the cracks that form in a refractory lining also may vary with the refractory composition and the thermal conditions. Refractory linings characterized by weaker bonds tend to form microcracks under stress while refractory linings characterized by stronger bonds tend to form macrocracks under stress. Macrocracking is particularly undesirable because it results from the failure of high strength bonds.
In addition to being unpredictable, cracking failures can be catastrophic. A macrocrack that extends completely through the lining from the hot face to the cold face (e.g., the steel shell side of a metal processing vessel) may allow molten metal and/or slag to reach the outer shell of the vessel by traveling through the crack. When this occurs, the molten materials can burn through the shell, which may result in extensive damage to equipment and/or injury to personnel. A bum-through of this type can cause long, unscheduled delays in the operation to make repairs to the lining, steel shell and structure, and any surrounding equipment.
Refractories also may be used in thermal insulation applications (in the metal processing field or otherwise) where repeated thermal shocks are expected. Such applications may include flue wall constructions and incinerators. Although erosion may occur in thermal insulation refractory applications in particularly corrosive environments, failure of thermal insulation refractories typically result from cracking caused by repeated thermal shocks.
Dry refractories, and particularly dry refractories that are installed using vibration to compact the dry refractory power, provide superior resistance to crack propagation compared to other types of conventional refractory linings such as castables, ramming materials, bricks, and refractory shapes. The superior crack resistance of dry vibratable refractory linings results from a unique bonding system that allows these linings to respond to the thermal conditions of the application by forming thermal bonds at controlled rates in predetermined temperature ranges. For example, in a metal containment application, the refractory lining responds to the thermal conditions of the associated molten metal vessel and any intrusions of molten metal and slag into the lining. The chemical and mineralogical compositions of dry vibratable refractories used in metal containment and heat/metal containment applications also may be selected to be resistant to specific types of metal and slags associated with particular processes.
An installed dry vibratable refractory initially exists in an unbonded state. In this unbonded state, it exhibits no brittle behavior. The unbonded dry refractory lining does not crack or fracture when subjected to external stresses but instead absorbs and distributes these stresses. As the unbonded installed refractory lining is exposed to heat, however, it begins to form thermal bonds. The region nearest the hot face tends to form strong thermal bonds. The strongly bonded refractory becomes dense and hard and is chemically and physically resistant to penetration by molten metal and slag.
The extent of the thermal bonding varies with the refractory composition and the thermal conditions present in a particular application. In some applications, essentially all of the refractory is expected to be strongly bonded and to exhibit brittle behavior. In other applications, the region furthest from the hot face is expected to remain in an unbonded or unsintered condition and the intermediate area is expected to form weak fritted thermal bonds. The refractory in the fritted and unsintered regions retains its fluid properties and forms an envelope that remains capable of absorbing mechanical and thermal stresses. Within this envelope, the strongly bonded refractory nearest the hot face may exhibit brittle behavior typical of conventional refractory compositions. However, this protective envelope may be degraded or even eliminated if the thermal conditions in the application cause bonding of the refractory in the fritted and unsintered regions.
The nature of the thermal bonding also varies with the refractory composition and the thermal conditions present in a particular application. Linings with weaker bonds tend to form microcracks under stress while linings with stronger bonds tend to form macrocracks under stress. As macrocracks form and molten metal and slag intrude into the refractory lining, the lining adjacent to the cracks respond to changes in thermal conditions and begin to form thermal bonds. As this cycle continues, the proportion of the refractory lining that exhibits brittle behavior progressively increases, driving the thermal plane of the lining toward the shell. If the lining has not failed or been taken out of service earlier as a result of erosion, eventually, the proportion of unbonded and weakly bonded refractory available to absorb and distribute stress is too small and failure of the lining results.
In view of the disadvantages of the prior art, a need exists for a dry refractory for metal/heat containment and thermal insulation applications that provides greater resistance to crack propagation, exhibits less brittle behavior when bonded, and has a longer service life.
It is an object of the invention to provide a dry refractory for metal/heat containment and thermal insulation applications that is resistant to crack propagation, and particularly macrocracking.
It is an object of the invention to provide a dry refractory for metal/heat containment and thermal insulation applications that exhibits less brittle behavior when the installed refractory has formed strong bonds in response to heat.
It is an object of the invention to provide a dry refractory for metal/heat containment and thermal insulation applications that provides a longer lining service life.